The present invention relates to medical diagnostic devices, and particularly to battery-powered pulse oximeters.
Diagnostic monitors are used to monitor various physiological parameters of a patient. In particular, such monitors are used for heart rate, respiration rate, blood pressure, temperature, and arterial oxygen saturation. Pulse oximeters, for example, illustrate the different aspects of such monitors, and are used as an example herein.
Pulse Oximeters are commonly used to monitor the level of oxygen in the blood of a patient. This is particularly critical during an operation, or during post-operation recovery. In addition, pre-delivery monitoring of the oxygen in a fetus provides important information. A typical oximeter directs light to the skin of a patient, with either transmitted or reflected scattered light being measured by a light detector. The amount of light detected will be diminished by the amount of light absorbed by the oxygen in the patient's blood. By using appropriate wavelengths of light emitters, and their known absorption characteristics along with appropriate mathematical algorithms, the oxygen saturation of a patient can be monitored.
Because of the need to quickly react to a change in a patient's condition, it is oftentimes important for a pulse oximeter to be in a continuous monitoring mode, with alarm limits set to generate an alarm in case the patient exceeds certain parameters. It is also desirable to be able to provide a portable pulse oximeter so that it can be moved from room to room without requiring it to be plugged in to a power outlet. In such a portable pulse oximeter, power consumption is of concern, and it is desirable to minimize the power consumption. Such a portable oximeter may not only consume power in performing the measurements, but might also transmit signals to a remote host computer for monitoring. In a portable oximeter, such transmissions may be done using wireless methods.
In one existing pulse oximeter, the Nellcor N-20, a snapshot mode is provided. In this mode, the oximeter can be turned on for a short period long enough to acquire a signal and provide a pulse oximeter reading, and then automatically shuts down. This method is useful primarily for taking a reading of a healthy or stable patient, and is not useful for a patient which requires continuing monitoring due to the patient's condition. When the snapshot mode button is pushed, five pulses are qualified and an oxygen saturation and pulse rate are displayed. No more measurements are made unless the snapshot button is pressed again. If the snapshot button is not pressed for 30 seconds, the N-20 automatically turns itself off. The N-20 also has an extended mode in which it continuously takes data and calculates and displays oxygen saturation and pulse rate.
The inventors of the present invention recognized that sleep-mode techniques used in other technologies could be imported into medical diagnostic devices, such as pulse oximetry, under the appropriate conditions. In particular, a sleep mode could be entered under carefully prescribed conditions for short periods while a patient's physiological state is stable.
Sleep-mode techniques have been used in other technologies, notably for lap-top computers which run on batteries. In a typical sleep-mode, power to certain components of the computer is turned off, or they are slowed down by reducing the clock speed, to reduce power consumption. Typically, these take advantage of the fact that a computer user is not always using the computer. Thus, for instance, circuitry in the computer may detect the absence of a keystroke for a certain amount of time, and enter a sleep mode in response. Enough circuitry is left on to detect an interrupt due to a keystroke, and the rest of the circuitry is awakened on such an occurrence. Certain microprocessors have a sleep or standby mode built in, with some microprocessors being able to be shut down completely, while others accept a vastly reduced clock speed. The microprocessor will automatically save the state of its registers and take any other action necessary to be able to resume from where it left off in its program.
Certain aspects of a computer system may be required to have power supplied to them constantly. For instance, DRAM memory is required to be periodically refreshed in order to save the memory contents. Other types of memory which are non-volatile, and do not require refreshing, are available. However, non-volatile memory is typically much more expensive, and thus there is a cost/power savings trade-off.
Sleep mode techniques have also been used in other technologies. For example, U.S. Pat. No. 4,716,463 discusses the use of batteries to keep a television powered during a power failure. Sleep mode is entered automatically upon detection of a power failure. U.S. Pat. No. 5,142,684 discusses the use of a sleep mode in a portable bar code reader. U.S. Pat. No. 4,924,496 discusses the use of a sleep mode in a telephone with incoming telephone call number display capability.
A number of patents discuss various uses of a sleep mode in an implantable device such as a pacemaker. Examples include U.S. Pat. No. 4,554,920, U.S. Pat. No. 4,561,442, and U.S. Pat. No. 4,856,524. Clearly, in an implantable device which is required to run on a battery, extending battery life is important so that another operation is not necessary to remove and replace the implanted device. Pacemakers can be put into a sleep mode for a variety of conditions. In particular, these patents discuss putting the pacemaker into a sleep mode during the refractory period, which is a period between heart beats when the heart muscle is non-responsive to electrical stimuli. U.S. Pat. No. 4,404,972 discusses not only implantable pacemakers, but also implantable devices for controlling bladder function, producing muscle contractions effective to combat scoliosis, to assist in countering pain-producing nerve impulses, and to control the infusion of various solutions into the body. These devices are all therapeutic, delivering material or energy to the body at controlled times. They do not collect data for diagnostic purposes, where the condition of the patient is unknown.
The '972 discusses a sleep mode in which the microprocessor is put to sleep, but not the other circuitry for detecting physiological events. The microprocessor can then be awakened either by a timer or upon the detection of a psychological event. When the microprocessor awakens, it can examine any events which may be stored or latched that occurred while the microprocessor was asleep.